1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an apparatus for positioning a user equipment and method thereof.
2. Discussion of the Related Art
Physical Structure of LTE
Generally, 3GPP LTE (3rd generation project partnership long term evolution) supports a type-1 radio frame structure applicable to FDD (frequency division duplex) and a type-2 radio frame structure applicable to TDD (time division duplex).
FIG. 1 shows a structure of a type-1 radio frame.
Referring to FIG. 1, a type-1 radio frame consists of 10 subframes. Each of the frames consists of 2 slots.
FIG. 2 shows a structure of a type-2 radio frame.
Referring to FIG. 2, a type-2 radio frame consists of 2 half frames. Each of the half frames consists of 5 subframes, a DwPTS (downlink pilot time slot), a guard period (GP), and an UpPTS (uplink pilot time slot). And, each of the subframes consists of 2 slots. The DwPTS is used for initial cell search, synchronization or channel estimation in a user equipment. The UpPTS is used for channel estimation and in matching uplink transmission synchronization of a user equipment. The guard period is a period for eliminating interference generated from uplink due to a multi-path delay of a downlink signal between uplink and downlink. In particular, irrespective of a type of radio frame, 1 subframe consists of 2 slots.
FIG. 3 shows a structure of an LTE downlink slot.
Referring to FIG. 3, a signal transmitted in each slot can be represented as a resource grid consisting of NRBDL NscRB subcarriers and NsymbDL OFDM (orthogonal frequency division multiplexing) symbols. In this case, the NRBDL indicates the number of resource blocks (RBs) in downlink (DL), the NscRB indicates the number of subcarriers constructing 1 RB, and the NsymbDL indicates the number of OFDM symbols in one downlink slot.
FIG. 4 shows a structure of an LTE uplink slot.
Referring to FIG. 4, a signal transmitted in each slot can be represented as a resource grid consisting of NRBULNSCRB subcarriers and NsymbUL OFDM (orthogonal frequency division multiplexing) symbols. In this case, the NRBUL indicates the number of resource blocks (RBs) in uplink (UL), the NscRB indicates the number of subcarriers constructing 1 RB, and the NsymbUL indicates the number of OFDM symbols in one uplink slot.
Resource element (RE) is a resource unit defined as indexes a and b within the UL/DL slot and indicates 1 subcarrier and 1 OFDM symbol. In this case, ‘a’ indicates an index on a frequency axis and ‘b’ indicates an index on a time axis.
FIG. 5 is a diagram for a structure of a DL subframe.
Referring to FIG. 5, maximum 3 OFDM symbols located in front part of a first slot within one subframe correspond to a control region allocated to a control channel. The rest of OFDM symbols correspond to a data region allocated to a physical downlink shared channel (PDSCH). For example, downlink control channels used by 3GPP LTE include PCFICH (Physical Control Format Indicator Channel), PDCCH (Physical Downlink Control Channel), PHICH (Physical Hybrid ARQ Indicator Channel) and the like.
Definition of MIMO
First of all, MIMO is an abbreviation of multiple-input multiple-output and indicates a scheme of raising data transceiving efficiency by adopting multiple transmitting antennas and multiple receiving antennas instead of using a single transmitting antenna and a single receiving antenna conventionally. In particular, MIMO is the technology of increasing capacity or enhancing performance using the multiple antennas in a transmitter or receiver of a wireless communication system. In the following description, the MIMO shall be named a multi-antenna.
The multi-antenna technology applies a technique of completing a message by gathering data fragments received via several antennas together instead of depending on a single antenna path to receive the message. Since the multi-antenna technology enhances a data rate within a specific range or extends a system range for a specific data rate, it is the next generation mobile communication technology that is widely applicable to a mobile communication terminal, a relay and the like. Many attentions are paid to the multi-antenna technology as a next generation technology to overcome a throughput limit of mobile communication on the verge of a critical situation due to expansion of data communication and the like.
FIG. 6 is a diagram for a configuration of a multi-antenna (MIMO) communication system.
Referring to FIG. 6, if the number of transmitting antennas and the number of receiving antennas are simultaneously incremented into NT and NR, respectively, channel transmission capacity theoretically increases in proportion to the number of antennas unlike the case of using a plurality of antennas in a transmitter or receiver only. Hence, a transmission rate is raised and frequency efficiency can be dramatically enhanced. The transmission rate according to the increase of channel transmission capacity can be theoretically raised by an amount resulting from multiplying a maximum transmission rate R0 in case of using a single antenna by an increase rate Ri shown in Formula 1.Ri=min(NT,NR)  [Formula 1]
For instance, in an MIMO communication system using 4 transmitting antennas and 4 receiving antennas, it is able to theoretically obtain a transmission rate 4 times greater than that of a single antenna system. Since this theoretical capacity increase of the multi-antenna system has been proved in the middle of 90's, many efforts are ongoing to be made to research and develop various technologies to provide substantial data transmission rate improvement. And, some of the technologies have been already reflected on the standards of the various wireless communications such as 3rd generation mobile communications, next generation wireless LAN and the like.
Regarding the multi-antenna relevant study trends, many efforts in various aspects are ongoing to be made to research and develop information theory related to multi-antenna communication capacity calculations and the like in various channel configurations and multiple access environments, radio channel measurement and modeling of multi-antenna systems, spatiotemporal signal processing for enhancements of transmission reliability and transmission rate, etc.
Channel Estimation
In a wireless communication system environment, fading occurs due to a multi-path time delay. A process for reconstructing a transmission signal by compensating signal distortion generated from abrupt environment change attributed to the fading is called channel estimation. In general, the channel estimation is performed using a signal known to both a transmitting side and a receiving side. In this case, the signal known to both of the transmitting side and the receiving side is called a pilot signal or a reference signal (hereinafter abbreviated RS).
In a wireless communication system using orthogonal frequency division transmission scheme, reference signals are classified into a type of allocating reference signals to all subcarriers and a type of allocating reference signals between data subcarriers.
In order to obtain a gain of channel estimation performance, a symbol including only reference signals such as a preamble signal is used. If a preamble signal is used, since density of reference signals is high, channel estimation performance can be improved better than that of the type of allocating reference signals between data subcarriers. Yet, since a transmission amount of data is reduced, the type of allocating the reference signals between the data subcarriers is used to increase the transmission amount of the data. If this type is used, the reference signal density is lowered. Therefore, the channel estimation performance is degraded. And, the demand for minimizing the degradation of the channel estimation performance is rising.
A receiver performs channel estimation using a reference signal in the following manner. First of all, since a receiver is aware of information of a reference signal, channel information between the receiver and a transmitter is estimated from a received signal. The receiver is able to correctly demodulate data transmitted by the transmitter using the estimated channel information value.
If a reference signal transmitted by a transmitter, channel information experienced by the reference signal in the course of transmission, a heat noise generated from a receiver, and a signal received by the receiver are set to p, h, n and y, respectively, the received signal y can be represented as ‘y=h·p+n’. In this case, since the receiver is already aware of the reference signal p, it is able to estimate channel information ĥ according to Formula 2.ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Formula 2]
In Formula 2, accuracy of a channel estimation value ĥ estimated using the reference signal p depends on a value {circumflex over (n)}. In order to estimate an accurate value ĥ, the {circumflex over (n)} should converge into 0. Hence, a channel should be estimated using a number of reference signals. If the channel is estimated using a number of reference signals, influence of the {circumflex over (n)} can be minimized.
UE-Specific Reference Signal Allocation Scheme in 3GPP LTE Downlink System
A structure of a radio frame applicable to FDD among the above described radio frame structures supported by 3GPP LTE is explained in detail as follows. First of all, 1 frame is transmitted in 10 msec. This frame consists of 10 subframes. And, one subframe is transmitted in 1 msec.
One subframe consists of 14 or 12 OFDM (orthogonal frequency division multiplexing) symbols. And, the number of subcarriers in one OFDM symbol is set to one of 128, 26, 512, 1024, 1536 and 2048 to use.
FIG. 7 is a diagram of a structure of a UE-specific (user equipment-specific) DL reference signal in a subframe when 1 TTI (transmission time interval) uses a normal cyclic prefix (CP) having 14 OFDM symbols.
Referring to FIG. 7, ‘R5’ indicates a UE-specific reference signal and ‘I’ indicates a position of an OFDM symbol in a subframe.
FIG. 8 is a diagram of a structure of a UE-specific DL reference signal in a subframe when 1 TTI (transmission time interval) uses an extended cyclic prefix (CP) having 12 OFDM symbols.
FIGS. 9 to 11 are diagrams of structures of UE-common DL reference signals for systems having 1, 2 and 4 transmitting antennas, respectively, when 1 TTI has 14 OFDM symbols.
Referring to FIGS. 9 to 11, R0 indicates a pilot symbol for a transmitting antenna 0, R1 indicates a pilot symbol for a transmitting antenna 1, R2 indicates a pilot symbol for a transmitting antenna 2, and R3 indicates a pilot symbol for a transmitting antenna 3. And, in order for eliminating interference with the rest of the transmitting antennas except the transmitting antenna, via which a pilot symbol is not transmitted, a signal is not carried on a subcarrier for which the pilot symbol of each of the transmitting antennas is used.
FIG. 7 and FIG. 8 show the structures of the UE-specific DL reference signals, each of which can be simultaneously used together with the UE-common DL reference signal shown in FIGS. 9 to 11. For instance, the UE-common DL reference signals shown in FIGS. 9 to 11 are used for the OFDM symbols 0 to 2 of a first slot in which control information is transmitted. And, the UE-specific DL reference signal is usable for the rest of the OFDM symbols.
Moreover, it is able to enhance channel estimation performance by reducing interference of pilot symbol received by a receiver from a neighbor cell in a manner that a DL reference signal per cell is transmitted by being multiplied by a predefined sequence (e.g., pseudo-random (PN) sequence, m-sequence, etc.). The PN sequence is applied by an OFDM symbol unit within one subframe. And, the PN sequence is differently applicable according to a cell ID, a subframe index, an OFDM symbol position and a user equipment ID.
For example, in case of the structure of 1Tx pilot symbol shown in FIG. 9, it can be observed that 2 pilot symbols of one transmitting antenna are used for a specific OFDM symbol including a pilot symbol. In case of the 3GPP LTE systems, there is a system including bandwidths of various types. In this case, the types include 6 RBs (resource blocks) to 110 RBs. Hence, the number of pilot symbols of one transmitting antenna in one OFDM symbol including a pilot symbol is 2×NRB. And, a sequence used by being multiplied by a downlink reference signal per cell should have a length of 2×NRB. In this case, the NRB indicates the number of RBs according to a bandwidth. And, a binary sequence or a complex sequence may be used as a sequence. In Formula 3, r(m) represents one example for a complex sequence.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                                                          ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB                              ma                ⁢                                                                  ⁢                x                                              -          1                                    [                  Formula          ⁢                                          ⁢          3                ]            
In Formula 3, NRBmax is the number of RBs corresponding to a maximum bandwidth and can be set to 110 according to the above description. Moreover, ‘c’ indicates a PN sequence and can be defined as a Gold sequence of length-31. In case of a UE-specific DL reference signal, Formula 3 can be represented as Formula 4.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                                                          ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB              PDSCH                                -          1                                    [                  Formula          ⁢                                          ⁢          4                ]            
In Formula 4, NRBPDSCH indicates the number of RBs corresponding to DL data allocated to a specific user equipment. Therefore, a length of sequence can vary according to an amount allocated to a user equipment.
The above described structure of the UE-specific DL reference signal can be transmitted as 1 data stream only. Since it is impossible to simply extend the structure, it is unable to transmit a plurality of streams. Therefore, the structure of the UE-specific DL reference signal needs to be extended to transmit a plurality of data streams.
User Equipment Positioning Method
The necessity of user equipment positioning is ongoing to raise according to various operations due to diverse application in real life environments. The user equipment positioning can be mainly classified into a GPS (global positioning system) based method and a terrestrial positioning based method.
The GPS based method measures a location of a user equipment using satellites. The GPS based method needs signals from at least 4 satellites. And, it is disadvantageous in that the GPS based method is not available for an indoor environment.
The terrestrial positioning based method measures a location of a user equipment using a timing difference between signals from base stations. The terrestrial positioning based method needs signals received from at least 3 base stations. The terrestrial positioning based method has positioning estimation performance poorer than that of the GPS based method but is available for almost every environment. The terrestrial positioning based method estimates a location of a user equipment using a synchronization signal or a reference signal generally. The terrestrial positioning based method is defined as the following terminology per standard.
In UTRAN (UMTS Terrestrial Radio Access Network), the terrestrial positioning based method is defined as OTDOA (Observed Time Difference of Arrival). In GERAN (GSM/EDGE Radio Access Network), the terrestrial positioning based method is defined as E-OTD (Enhanced Observed Time Difference). In CDMA2000, the terrestrial positioning based method is defined as AFLT (Advanced Forward Link Trilateration).
FIG. 12 is a diagram of an example for downlink OTDOA as a sort of a terrestrial positioning based method used by the 3GPP standard.
Referring to FIG. 12, since a current user equipment uses a reference clock with reference to a subframe transmitted by a current serving cell, signals received from neighbor cells are received with OTDOAs different from each other.
FIG. 13 is a diagram of an example for a user equipment positioning method using OTDOA.
Referring to FIG. 13, a location of a user equipment can be calculated by solving a linear equation using Taylor series expansion (cf. Y. Chan and K. Ho, “A simple and efficient estimator for hyperbolic location,” IEEE Trans. Signal Processing, vol. 42, pp. 1905-1915, August 1994).
The above mentioned user equipment positioning method can be normally performed via Common Reference Signal (CRS) or Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS). Alternatively, the user equipment positioning method can be performed in a manner of defining Positioning Reference Signal (PRS) dedicated to LCS (LoCation Service).
By defining a positioning subframe for LCS, data scheduling is not performed on a corresponding positioning subframe and a reference signal is transmitted only. The positioning subframe can be defined through scheduling in normal subframe or by configuring MBSFN subframe (e.g., periodicity of 80 ms or 320 ms). Es/It is increased by extending inter-cell reuse through the above method, whereby measurement of a neighbor cell is enabled. In this case, the ‘Es’ indicates signal energy of a specific signal. The ‘It’ indicates power spectral density of an interference signal and is generally called SINR.
The LCS requirements regulated by Federal Communications Commission (FCC) E911 (Enhanced 911) are shown in Table 1 (cf. FCC 99-245, “Revision of the Commission's Rules to Ensure Compatibility with Enhanced 911 Emergency Calling Systems”).
TABLE 1Solutions67% of Calls95% of CallsHandset-Based 50 meters150 metersNetwork-Based100 meters300 meters
Yet, in order to meet the above requirements according to a cell deployment scenario, multiple subframe averaging should be performed. To acquire a gain of the multiple subframe averaging, periodicity of a positioning subframe needs to be appropriately set. For instance, if a periodicity of a reference signal (RS) sequence is defined as 10 ms and a periodicity of a positioning subframe is 80 ms, effect of interference averaging does not exist.
Therefore, it is necessary to set a periodicity of a reference signal (RS) sequence and a periodicity of a positioning subframe to obtain interference averaging effect.